The Justice Department said Thursday it is refusing to let a key lawyer testify to the House oversight committee on the criminal investigation into the IRS, saying that to let her brief Congress could potentially skew its probe.

But oversight committee Republicans said blocking lawyer Barbara Bosserman from testifying only makes the Justice’s investigation look more partisan.

After an internal audit last year revealed the IRS was unfairly targeting tea party groups for intrusive scrutiny and blocking their applications for tax-exempt status, Attorney General Eric H. Holder Jr. called for a criminal probe into the tax agency.

Eight months later, the probe has shown few public signs of progress, and many of the tea party victims say they still haven’t heard from the FBI or Justice Department lawyers.

House Republicans said they were concerned about the direction of the probe, and Rep. Jim Jordan, Ohio Republican and a subcommittee chairman on the House oversight committee, asked Ms. Bosserman to testify at a hearing next week.

The GOP has identified her as the lead lawyer on the investigation, and they have questioned her role, given her history as a significant political donor to President Obama’s 2008 and 2012 campaigns.

Mr. Holder has denied Ms. Bosserman is the leader of the investigation and in a new letter to Mr. Jordan on Tuesday Deputy Attorney General James M. Cole, Mr. Holder’s deputy, said Ms. Bosserman won’t be testifying, nor will the department let anyone else appear.